In recent years, along with the progress in portable or codeless equipments, a demand is mounting for a non-aqueous electrolyte secondary battery which is small in size and light in weight and has a high energy density. As an active material for a non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal, such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Especially, a lithium secondary battery employing a lithium-cobalt composite oxide (LiCoO2) as a cathode active material and employing a lithium alloy or a carbon such as graphite or carbon fiber as a negative electrode, provides a high voltage at a level of 4 V and is widely used as a battery having a high energy density.
However, in a case of the non-aqueous type secondary battery employing LiCoO2 as a cathode active material, further improvements of capacity density per a unit volume of a positive electrode layer and safety, have been desired, and there have been such problems as a problem of deterioration of cyclic properties that the battery discharge capacity gradually decreases as a charge/discharge cycle is repeated, a problem of weight capacity density, or a problem that the decrease of discharge capacity is significant at a low temperature.
In order to solve these problems, Patent Document 1 reports stabilization of crystal lattice of lithium-cobalt composite oxide and improvement of performances by substituting a part of cobalt element by elements such as manganese or copper by a so-called solid phase method in which raw material components are blended and fired in a state of solid phase. However, in this solid phase method, it was confirmed that although cyclic properties can be improved by the effect of the substituting elements, the thickness of the battery gradually increases as the charge/discharge cycle is repeated.
Further, Patent Document 2 reports improvement of performances of lithium-cobalt composite oxide by substituting a part of cobalt element by an element such as magnesium by a coprecipitation method. However, in this coprecipitation method, although more uniform substitution of element is possible, there are problems that the type or the concentration of substituting elements is limited and it is difficult to obtain a lithium-cobalt composite oxide having expected performances.
The present inventors discovered earlier that e.g. a lithium-cobalt composite oxide employing a raw material obtainable by a method of impregnating a solution containing a metal such as aluminum, magnesium or zirconium and containing a carboxylic acid and/or hydroxyl group, into a raw material powder of a compound of a transition metal such as cobalt, has excellent electrode performances of lithium secondary battery, and they proposed this method in Patent Document 3. However, it has become clear that e.g. a lithium-cobalt composite oxide obtained by such a method, has the above-mentioned high-performance electrode characteristics when they are produced in a small-sized lot, but when they are produced in an industrial production size, there occurs a problem that a lithium-cobalt composite oxide obtained does not exhibit sufficient performance of an electrode, and there is a problem in productivity at a time of mass production.
Patent Document 1: JP-A-5-242891
Patent Document 2: JP-A-2002-198051
Patent Document 3: WO2004/088776